1. Field of the Invention
The present invention relates to a receiver unit having a synchronous pull-in circuit. For example, a receiver unit of a modulator and demodulator (MODEM) contains a synchronous pull-in circuit which is operative to pull an internal clock of the receiver unit to a timing signal contained, as a clock, in the received signal at the receiver unit. The pull-in operation is performed such that the internal clock is synchronized in both phase and frequency with the timing signal. Particularly, the present invention concerns an improved synchronous pull-in circuit able to complete the pull-in operation in a very short time.
2. Description of the Related Art
In recent years, it has been a common practice to use existing telephone lines for data transmission. Existing telephone lines are usually analog lines in form. Therefore, it is necessary to employ a MODEM which modulates digital data signals into corresponding analog data signals and demodulates the latter signals into the former.
The problem is that the transmission characteristics of the telephone lines to which the MODEM receiver unit is connected are, in general, not the same. Due to this fact, a pull-in operation is essential for each MODEM receiver unit. A pull-in operation is usually achieved by, first, sending a so-called training signal from a transmitter side to the receiver unit every time data transmission is to be started. The internal clock of the MODEM receiver unit is then synchronized with the timing signal contained in the training signal. After the pull-in operation, the MODEM receiver unit can start inherent data processing correctly in response to the successive received signal.
During this pull-in operation, other pull-in operations are sometimes also performed in parallel, such as for equalization of an automatic equalizer (EQL), automatic gain control (AGC), and in carrier automatic phase control (CAPC). Through the synchronous pull-in operations, as mentioned previously, the internal clock of the MODEM receiver unit is synchronized in phase and frequency with the timing signal, i.e., baud rate clock, contained in the received signal at the MODEM receiver unit. During the reception of the training signal, the pull-in operation regarding the phase is especially important.
In a prior art technique for a synchronous pull-in operation regarding the phase, a jump in the phase is executed. In short, the difference in phase between the internal clock and the timing signal is cancelled by the jump in the phase of the internal clock.
The problem in the prior art pull-in technique is that an undesired transient response is produced for a while immediately after the execution of the jump in phase. The thus produced transient response has some inconveniences. First, it is difficult to increase the data transmission efficiency by shortening the timing signal duration. Such shortening is not, in actuality, possible due to the presence of the undesired transient response in the training signal duration, and therefore the duration is necessarily extended for a duration equal to that of the transient response. Second, in a case where the training signal duration is required to be shorter than a prescribed value currently used there is a possibility that the synchronous pull-in operation cannot be completed in the training signal duration due to the presence of the transient response.